In the past 50 years since the demonstration of the first silicon transistor there has been a continuous evolution in semiconductor integrated circuits from the first Small-Scale Integration (SSI) with only a few transistors the semiconductor industry has grown to dominate many aspects of human society. Today compact wireless portable devices harbor microprocessors with more processing power than state of the art desktop computers 30 years ago and provide seamless integration of multiple applications such as office applications, gaming, electronic communications (wireless, text, email), and Internet browsers as well as supporting operation to multiple wireless standards. Hewlett-Packard's first laptop computer in 1984 the HP-110 and IBM's famous XT desktop computer in 1981 exploited Intel 8088 processors operating at 10 MHz with 256 kB of RAM and 10 MB hard-disk drive (HDD) and were considered too expensive for consumers and business machines. Today teenagers replace their Apple iPhone every couple of years as an easily disposed off high volume consumer item with 1 GHz process (100 times higher clock), 512 Mb RAM (2000 times higher) and 32 GB high speed flash HDD (3000 times higher, smaller, faster, less power).
Silicon digital circuits have evolved through successive generations of SSI, MSI, LSI, VLSI, ULSI, WSI (wafer scale integration), SOC (system-on-a-chip) and 3D-IC to provide tens of billions of transistors. This rapid development of the semiconductor industry has had a profound impact in our society where kindergarten children play with toys with more processing power than NASA's Apollo lunar landers, where children learn, communicate, and are entertained increasingly with compact portable electronics, and adults in nearly every aspect of their life cannot work, cook, travel, entertain without a microprocessor being involved somewhere.
In 2009 the global sales for semiconductor devices for entertainment, communications, transportation, medical systems was approximately US$226 billion, with US based businesses accounting for US$112 billion of that. But the industry features a number of distinct characteristics that position it uniquely in the economy and in the global competitive arena. These include:                The semiconductor industry is widely recognized as a key driver for economic growth in its role as a multiple lever and technology enabler for the whole electronics value chain. In other words, whilst the worldwide base semiconductor market was approximately US$200 billion in 2004, the industry enabled the generation of some US$1,200 billion in electronic systems business and US$5,000 billion in services, representing close to 10% of worldwide GDP;        The need for high degrees of flexibility and innovation in order to constantly adjust to the rapid pace of change in the market. Many products embedding semiconductor devices often have a very short life cycle. At the same time, the rate of constant price-performance improvement in the semiconductor industry is staggering. As a consequence, changes in the semiconductor market not only occur extremely rapidly but also anticipate changes in industries evolving at a slower pace. Yet another consequence of this rapid pace is that established market strongholds can be displaced all too quickly.        
Within this semiconductor industry silicon dominates but many aspects of our electronic systems, devices, and infrastructure would not exist without the low cost availability of high performance devices and circuits exploiting binary semiconductors such as silicon germanium (SiGe, for high frequency RF circuits) and gallium arsenide (GaAs, for high frequency RF circuits) or the myriad of compound semiconductors from the quaternary semiconductor indium gallium arsenide phosphide (InGaAsP, for high speed electronics, optoelectronic devices for multi-gigabit optical communications, LEDs etc). In addition to enabling the infrastructure of telecommunication networks today from 100 Gb/s backbone networks to Fibre-to-the-Home (FTTH) in the optical domain and 3G, 4G, WiFi, WiMAX in the wireless domain such semiconductors and a range of other materials provide the key to addressing a wide range of issues ranging from sustaining the drive for increased integration and functionality through to the fundamental sustaining of our expectations of heat and light.
Solar Cells: Developing an abundant renewable energy source is required to mitigate climate change, maintain our standard of living in the developed countries and improving the standard of living in the large parts of the world we label as developing. Currently, geographically-limited, clean energy sources, such as wind, geothermal and hydro power, are cost-competitive with grid energy generated from fossil or nuclear fuels. Solar-derived, clean energy, on the other hand, is universally available but is not yet cost-competitive. Because of its availability and abundance, solar energy has great potential to reduce climate change; but, its high cost/Watt is the major hurdle to its adoption. First generation photovoltaics (PVs) produced energy at a cost of about $0.30/kWh with efficiencies around 14% and were constructed on single-crystalline, Si substrates. Second generation, thin-film-based PVs grown on amorphous substrates are less efficient, around 8%; however, because of their reduced costs, typically they produce energy at a lower cost/Watt, about $0.10/kWh. Second generation PVs are less efficient because of their poly-crystalline or amorphous microstructures which increase recombination rates and decrease efficiency.
Third generation solar cells are expected to have high efficiencies, associated with high-quality semiconductors of multiple materials to cover increased spectral range, at reduced costs, associated with amorphous substrates, and will better compete with grid electricity, ˜$0.04/kWh. Third generation PVs will require therefore the production of high-quality semiconductors on inexpensive, large substrates. These third generation multi-junction PVs maximize energy production by converting high-energy photons to excitons using large-bandgap semiconductors and longer-wavelength photons to excitons with smaller-bandgap materials and are typically based on the Ge/GaAs lattice constant, partially because a large range of bandgaps are available at or near this lattice constant, see for example J. F. Geisz et al in “40.8% Efficient Inverted Triple-Junction Solar Cell with Two Independently Metamorphic Junctions” (Applied Phys. Lett., Vol. 93, No. 12, 123505, 3 pages). However, both Ge and GaAs substrates are small and expensive making these highly-efficient PVs (>40%) expensive and unable to enjoy economies-of-scale. In addition, this lattice constant does not have all the needed bandgaps to make the most efficient PVs, around 0.7, 1.2, and 1.8 eV, see for example A. Marti et al in “Limiting Efficiencies for Photovoltaic Energy Conversion in Multigap Systems” (Solar Energy Materials and Solar Cells, Vol. 43, No. 2, pp 203-222, 1996).
To achieve cost parity with the electrical grid, both decreased PV price and increased efficiency are needed. To date, most technologies have focused on either the efficiency (e.g. multi-junction PVs, see Geisz and Marti supra for example) or the cost (e.g. PVs based on amorphous substrates, see K. Yamamoto et al in “Novel Hybrid Thin Film Solar Cell and Module” (Proc. 3rd World Conference Photovoltaic Energy Conversion, pp 2789-2792, 2003)) to impact the $/Watt figure of merit. However, there are few technologies that can address both the numerator and the denominator because, heretofore, high-efficiency and low-cost have been largely orthogonal goals. This arises as high efficiency PVs require high-quality, small and expensive semiconductor substrates; while low cost PVs, required large amorphous substrates, upon which high-quality materials were unavailable. Attempts to address this with direct growth of planar semiconductors on amorphous substrates typically fail as they lead to amorphous or poly-crystalline materials because of the coalescence of crystals from many uncontrolled, randomly-oriented nucleation events during the growth (see for example C. V. Thompson et al in “Texture Development in Polycrystalline Films” (Mat. Sci. and Eng. B, Vol. 32, No. 3, pp 211-219)). Amorphous and poly-crystalline microstructures increase minority carrier recombination thereby reducing PV efficiency. It would be beneficial for such broad spectrum high efficiency and low cost solar cells to therefore provide a method of controlling the growth location and nucleation of crystals to ensure pre-defined areas of single-crystalline semiconductor material are formed on large amorphous substrates.
Solid State Lighting: The establishment 100 years ago of tungsten filament lamps fundamentally shifted how people live, work, play. However, the efficiency of such light sources is woefully low. For example a 60 W incandescent light is only 2.1% efficient, a quartz halogen only 3.5%, in terms of generating light within the visible spectrum of the human eye. Accordingly today there is a massive worldwide campaign to have incandescent lights replaced wherever possible by compact fluorescent lights (CFL) which have an efficiency of approximately 22% thereby reducing energy consumption significantly. However, whilst CFLs provide an immediate and visible statement by Governments and other organizations worldwide that they are addressing global climate change, environmental issues etc they are not a panacea and have many disadvantages including health, safety, and environmental issues from their mercury content, UV emissions degrading some materials, flicker affecting individuals with conditions including autism, epilepsy, lupus, chronic fatigue syndrome, and vertigo, radio interference, operating temperature where efficiency drops with increasing/decreasing temperature from room temperature, non-operation at below freezing, low-luminance requiring long tubes and limiting power output, dimming, and recycling through the phosphor and mercury.
In contrast a monochromatic solid state light source within the visible wavelength range can achieve in principle an efficiency approaching 100%. Additionally such solid state light sources should also reduce consumption of precious metals, reduce recycling as well as address health and safety issues. Beneficially solid state light sources by virtue of their small size, low weight, and low voltage operation can also be employed in a wide range of situations where incandescent or CFL lights cannot. But today solid state lighting sales accounts for only approximately 2% of the global lighting market as the majority of the lighting market remains inaccessible despite the considerable research effort and investment that have been expended in the past decade.
This arises due to the challenges in realizing suitable LED technologies and devices using conventional quantum well structures, these including the relatively low internal quantum efficiency of these structures, the low light extraction efficiency realized, and relatively high device fabrication costs. Additionally to achieve a “white” LED today these devices generally employ a phosphor-conversion scheme, which sets the ultimate quantum efficiency of white LEDs to below 65%. In this regard, the high luminescence efficiencies, low fabrication costs, and processibility of semiconductor nanostructures, including quantum dots and nanowires, have made them promising candidates for future lighting devices and the subject of considerable research and development.
Whilst the ternary AlGaAs and quaternary InGaAsP/InGaAlP material systems have been subject to substantial development for infra-red/red light sources for optical communications and displays developments for LEDs within the remainder of the visible spectrum have been scattered across multiple materials including nitrides of group III elements including aluminum, gallium and boron. In contrast InN exhibits the highest electron mobility (4400 cm2V−1s−1 at 300 K), the smallest effective mass, and the highest saturation velocity of the group III nitrides making it an excellent candidate for next generation of nanophotonic and nanoelectronic devices, including chip level nano scale lasers and high-speed field effect transistors. Additionally, the band gap of InN at approximately 0.7 eV (1750 nm) when compared with GaN at approximately 3.3 eV (370 nm) means that the ternary alloy InGaN can be continuously tuned from ˜0.7 to 3.3 eV, matching almost perfectly the solar spectrum and visible spectrum of the human eye. Therefore, InGaN has also emerged as a promising material for future both high-efficiency full solar spectrum solar cells and high-efficiency white light sources.
It should be apparent to one skilled in the art that in order to provide such broad spectrum optical emitters/absorbers that they should be structured so that the material at the front of the solar cell absorbs the shortest wavelengths and progressively longer wavelengths are absorbed by layers within the solar cell towards the substrate. As such, the material should grade from InxGa1-xN where x≈1 to InyGa1-yN where y≈0. As such it is necessary to grow InN structures onto the substrate of the solar cell which may be plastic, ceramic, or amorphous silicon for example. As such it would be beneficial for such low cost high efficiency solid state light sources to provide a method of controlling the growth location and nucleation of crystals to ensure pre-defined areas of single-crystalline semiconductor material are formed on large amorphous substrates.
MEMS and Sensors: Microelectromechanical systems (MEMS) are small integrated devices or systems that combine electrical and mechanical components. The components can range in size from the sub-micrometer level to the millimeter level, and there can be any number, from one, to few, to potentially thousands or millions, in a particular system. Historically MEMS devices have leveraged and extended the fabrication techniques developed for the silicon integrated circuit industry, namely lithography, doping, deposition, etching, etc. to add mechanical elements such as beams, gears, diaphragms, and springs to silicon circuits either as discrete devices or in combination with integrated silicon electronics. Whilst the majority of development work has focused on silicon electronics additional benefits may be derived from integrating MEMS devices onto other existing electronics platforms such as silicon germanium (SiGe), gallium arsenide and, indium phosphide for RF circuits and future potential electronics platforms such as organic based electronics, nanocrystals, etc whilst supporting the drive towards compact hybridly integrated or SOC implementations.
Whilst today MEMS device applications include inkjet-printer cartridges, accelerometers, miniature robots, micro-engines, locks, inertial sensors, micro-drives, micro-mirrors, micro actuators, optical scanners, fluid pumps, transducers, chemical sensors, pressure sensors, and flow sensors. New applications are emerging as the existing technology is applied to the miniaturization and integration of conventional devices. These systems can sense, control, and activate mechanical processes on the micro scale, and function individually or in arrays to generate effects on the macro scale. The micro fabrication technology enables fabrication of large arrays of devices, which individually perform simple tasks, or in combination can accomplish complicated functions. Such systems incorporating MEMS will also increasing include the integration of chemical and biological sensors as well as pushing the envelope of performance of electronics and micro-mechanical design. For example, a GaAs RF oscillator may provide an enhanced clock through down conversion for a high speed silicon IC than multiplying up a low frequency silicon oscillator or requiring an external crystal oscillator.
In some MEMS structures it would be beneficial to provide amorphous materials, such as silicon carbide (SiC) for example into MEMS devices and systems. SiC offers improved mechanical properties such as higher acoustic velocity, high fracture strength and desirable tribological properties as well as having an ability to sustain higher temperatures and resist corrosive and erosive materials. Hence, it would be beneficial for some applications to form silicon MEMS devices upon an amorphous SiC substrate removing temperature or chemical constraints from packaging all-silicon MEMS devices. Equally forming localized GaAs, InP, or other semiconductor materials within predetermined regions of amorphous silicon substrates may provide enhanced MEMS and/or system functionality including for example electronic or optoelectronic elements.
Integration of Lattice Mismatched Materials: As evident from considerations of solid state lighting, MEMS, PVs there is significant benefit from integrating either materials with mismatched lattices, e.g. GaAs and Si, or forming crystalline materials on amorphous materials, e.g. InN on α-Si. However, currently the integration of lattice mismatched materials is a significant challenge. Approaches adopted to date have included bonding, such as epitaxial lift-off through sacrificial layers within the semiconductor structure, see for example P. Demeester et al in “Epitaxial lift-off and its applications” (Semi. Sci. & Tech., Vol. 8, No. 6, pp 1124-), R. Moug et al in “Development of an Epitaxial Lift-Off Technology for II-VI Nanostructures using ZnMgS Se Alloys” (Microelect. J., Vol. 30, No. 3, pp 530-532) and even semiconductors to ferroelectrics, see for example A. C. O'Donnell et al in “Integration of GaAs MESFETs and Lithium Niobate Optical Switches using Epitaxial Lift-Off” (Elect. Lett., Vol. 26, No. 15, pp 1179-181), and wafer bonding, see for example M. Alexe et al in “Wafer Bonding—Applications and Technology” (Springer-Verlag ISBN 3-540-21049-0) and S. S. Iyer et al in “Silicon Wafer Bonding Technology for VLSI and MEMS Applications” (Published by INSPEC ISBN 0 85296 039 5).
These approaches being employed to address the disadvantages of direct growth with mismatched lattice semiconductors, see for example P. Demeester et al in “Relaxed lattice-mismatched growth of III-V semiconductors” (Prog. Crystal Growth and Characterization of Materials, Vol. 22, Nos. 1-2, pp 53-141) and D. Gerthsen et al in “Structural Properties of Lattice-Mismatched Compound Semiconductor Heterostructures” (Adv. Solid State Phys, Volume 34/1994, pp 275-295). Alternatively within direct growth adding additional layers to act as buffer layers has been exploited, but as with direct growth the acceptable mismatches that can be accommodated are low unless extremely thick buffer layers are employed which in many instances destroy the function of the electrical or optical device, see for example P. M. Mooney et al in “Si Ge Technology: Heteroepitaxy and High-Speed Microelectronics” (Ann. Rev. Mat. Sci. Vol. 30, pp 335-362) and J. E. Ayers in “Heteroepitaxy of Semiconductors: Theory, Growth and Characterization” (CRC Press 2007).
Typically direct growth is preferred because graded buffers can require microns of growth to get low defect densities and bonding is both, expensive and difficult. In the direct growth method (and graded buffer method, for that matter) a lattice-mismatched film is grown onto the surface of a substrate which provides the crystalline template for the film. However, direct growth also yields the poorest quality material, wherein threading dislocation densities generally increases with lattice mismatch and can exceed 108/cm2 for the large lattice-mismatches within devices such as multi-junction PVs. In addition to threading dislocations the as grown films exhibit increased stress and cracking.
Accordingly it would be beneficial to provide a means of growing high quality crystalline films of semiconductors on substrates or other epitaxially grown layers wherein the underlying layer/substrate either exhibits a high lattice mismatch between the two crystalline materials or the crystalline semiconductor and an amorphous material. As such there is disclosed a novel, metal-catalyzed growth process that occurs laterally rather than vertically as commonly occurs in most epitaxial processes based upon techniques such as LPE, MOVPE, MBE, MOCVD etc. With this technique the location of crystal nucleation is controlled from the catalyst location as well as the formation of grain boundaries, typical when crystals grow together, is prevented. Such engineered crystal nucleation and lateral growth provides for an enablement of devices such as PVs, solid state lighting, high speed electronics and optoelectronics by allowing large amorphous substrates to be tiled with single-crystalline semiconductors within which these devices are subsequently formed through photolithography, etching, doping, metallization etc. Further by providing increased flexibility in the layers within such devices essentially another degree of design freedom is given to circuit designs to produce high efficiency, low cost structures through enabling the growth of high-quality, lattice-mismatched semiconductors.